The Great Barrier Reef
38
self-seeded and connected with other reefs located
‘upstream’. In areas where the reefs are scattered at
high density, the currents are steered around the
dense reef matrix instead of flowing through the
reef matrix, akin to a sticky water effect; reefs within
the dense matrix are highly self-seeded and little
connected with reefs outside. Recent evidence indicates that self seeding by recruit fishes can be as
high as 30% to 60% and it is likely that the behavioural abilities of presettlement fishes as well as current influence connectivity.
N FRESHWATER INPUT, CYCLONES AND MUD
Riverine input to the GBR is substantial and it has its
greatest impact near-shore (within 10 km of the mainland) and in and around what would be considered ‘inner’ reefs of the GBR. The river plumes also impact
mid-shelf and outer reefs in the Cairns area (Fig. 4.7 and
4.8). Patterns of rainfall vary along the GBR. Largest
rainfall occurs north of Hinchinbrook Island (near
Tully). There is a distinct seasonal pattern to rainfall
where there is a rainy season that extends from c.
December to April. Major perennial rivers such as the
Normandy and Daintree are in this area (see Chapter 11).
Muddy marine snow flocs as large as several millimetres are formed and carried in suspension by the
aggregation of mucus and mud, and they support
intense biological activity. The mucus is formed by
bacteria and plankton, and this is enhanced by nutrients from land runoff. The mud is brought in from
river plumes, wave resuspension and bottom-tagging
nepheloid layers. These aggregates settle in quiescent
areas in calm weather, as controlled by the oceanography, and being very sticky can readily smother
coral polyps and organisms living on coral reefs. This
mud can later be resuspended by waves and is carried away but the reef damage has been done in the
meantime.
Freshwater input has a great influence on the biology of inshore waters. In many parts of the world it has
been demonstrated that catch and recruitment rates of
prawns and fishes vary with freshwater input. Recruitment rates usually go up with input of freshwater.
Changes in freshwater runoff may happen through
anthropogenic alteration of catchments and through
changes in rainfall that relate to global warming (see
Box 4.2). There is concern that an increase in runoff and
nutrients could affect inner shelf reefs through phase
shifts (i.e. from coral to algae) and the survival of
crown-of-thorns starfish (COTS) larvae.
The wet season is the same as the cyclone season
and river floods often result from cyclones. Cyclones
can thus be very destructive because of the huge input
of sediment and freshwater input to the GBR and also
of wave height on reefs. Physical destruction can
be considerable and the swath of damage is asymmetrical with respect to the position of the eye of the
cyclone. Cyclones rotate clockwise in the southern
hemisphere and anticlockwise in the northern hemisphere as typhoons (Asia) or hurricanes (America).
The impact of cyclones on the GBR is greater on the
southern side of the eye due to the greater fetch from
the open ocean and the larger wave height. The destruction of reef habitat and death of organisms can be
substantial. Storm swell can blast large chunks of coral
(including large Porites) onto the reef flat. Damage
will usually only be to the windward side of reefs, but
life on coastal fringing reefs can be all but obliterated.
Great changes in habitat type (e.g. from a species rich
assemblage of live coral to coral rubble) will have a
great influence on local species diversity of most taxa
(see Chapter 5).
ADDITIONAL READING
Biological oceanography
Burgess, S. C., Kingsford, M. J., and Black, K. P. (2007).
Influence of tidal eddies and wind on the distribution of presettlement fishes around One Tree Island,
Great Barrier Reef. Marine Ecology Progress Series
341, 233–242.
Glynn, W. (1988). El Nino-Southern Oscillation 1982–
1983: Nearshore population, and ecosystem responses. Annual Review of Ecology and Systematics 19,
309–346.
Kingsford, M. J., Wolanski, E., and Choat, J. H. (1991).
Influence of tidally induced fronts and Langmuir
circulations on distribution and movements of
38
self-seeded and connected with other reefs located
‘upstream’. In areas where the reefs are scattered at
high density, the currents are steered around the
dense reef matrix instead of flowing through the
reef matrix, akin to a sticky water effect; reefs within
the dense matrix are highly self-seeded and little
connected with reefs outside. Recent evidence indicates that self seeding by recruit fishes can be as
high as 30% to 60% and it is likely that the behavioural abilities of presettlement fishes as well as current influence connectivity.
N FRESHWATER INPUT, CYCLONES AND MUD
Riverine input to the GBR is substantial and it has its
greatest impact near-shore (within 10 km of the mainland) and in and around what would be considered ‘inner’ reefs of the GBR. The river plumes also impact
mid-shelf and outer reefs in the Cairns area (Fig. 4.7 and
4.8). Patterns of rainfall vary along the GBR. Largest
rainfall occurs north of Hinchinbrook Island (near
Tully). There is a distinct seasonal pattern to rainfall
where there is a rainy season that extends from c.
December to April. Major perennial rivers such as the
Normandy and Daintree are in this area (see Chapter 11).
Muddy marine snow flocs as large as several millimetres are formed and carried in suspension by the
aggregation of mucus and mud, and they support
intense biological activity. The mucus is formed by
bacteria and plankton, and this is enhanced by nutrients from land runoff. The mud is brought in from
river plumes, wave resuspension and bottom-tagging
nepheloid layers. These aggregates settle in quiescent
areas in calm weather, as controlled by the oceanography, and being very sticky can readily smother
coral polyps and organisms living on coral reefs. This
mud can later be resuspended by waves and is carried away but the reef damage has been done in the
meantime.
Freshwater input has a great influence on the biology of inshore waters. In many parts of the world it has
been demonstrated that catch and recruitment rates of
prawns and fishes vary with freshwater input. Recruitment rates usually go up with input of freshwater.
Changes in freshwater runoff may happen through
anthropogenic alteration of catchments and through
changes in rainfall that relate to global warming (see
Box 4.2). There is concern that an increase in runoff and
nutrients could affect inner shelf reefs through phase
shifts (i.e. from coral to algae) and the survival of
crown-of-thorns starfish (COTS) larvae.
The wet season is the same as the cyclone season
and river floods often result from cyclones. Cyclones
can thus be very destructive because of the huge input
of sediment and freshwater input to the GBR and also
of wave height on reefs. Physical destruction can
be considerable and the swath of damage is asymmetrical with respect to the position of the eye of the
cyclone. Cyclones rotate clockwise in the southern
hemisphere and anticlockwise in the northern hemisphere as typhoons (Asia) or hurricanes (America).
The impact of cyclones on the GBR is greater on the
southern side of the eye due to the greater fetch from
the open ocean and the larger wave height. The destruction of reef habitat and death of organisms can be
substantial. Storm swell can blast large chunks of coral
(including large Porites) onto the reef flat. Damage
will usually only be to the windward side of reefs, but
life on coastal fringing reefs can be all but obliterated.
Great changes in habitat type (e.g. from a species rich
assemblage of live coral to coral rubble) will have a
great influence on local species diversity of most taxa
(see Chapter 5).
ADDITIONAL READING
Biological oceanography
Burgess, S. C., Kingsford, M. J., and Black, K. P. (2007).
Influence of tidal eddies and wind on the distribution of presettlement fishes around One Tree Island,
Great Barrier Reef. Marine Ecology Progress Series
341, 233–242.
Glynn, W. (1988). El Nino-Southern Oscillation 1982–
1983: Nearshore population, and ecosystem responses. Annual Review of Ecology and Systematics 19,
309–346.
Kingsford, M. J., Wolanski, E., and Choat, J. H. (1991).
Influence of tidally induced fronts and Langmuir
circulations on distribution and movements of